A TiO2-modified carbon nanotube-supported palladium single-atom catalyst and its preparation method and application

Abstract

The present invention discloses a TiO2-modified carbon nanotube-supported palladium single-atom catalyst, its preparation method, and application, belonging to the technical field of heterogeneous solid catalysts. The present invention uses a hydrothermal synthesis method to in-situ grow TiO2 on CNTs to form a TiO2-CNT heterojunction. Pd single atoms are then loaded onto the TiO2-CNT heterojunction through mechanical activation. After high-temperature calcination, a green, economical, stable, and directional catalytic TiO2-modified carbon nanotube-supported palladium single-atom catalyst is obtained. The palladium metal content is 0.18-0.20 wt % and the specific surface area is 210-215 cm 2 The catalyst of the present invention has a simple preparation method, readily available raw materials, simple and mild conditions, and low production cost. It exhibits excellent catalytic conversion efficiency for cinnamaldehyde, achieving high cinnamaldehyde conversion rates and phenylpropionaldehyde yields during the reaction. Furthermore, the catalyst of the present invention can be recycled multiple times, maintaining excellent catalytic conversion efficiency after three uses, and has promising application prospects for the efficient conversion of cinnamaldehyde to produce phenylpropionaldehyde.

Description

Technical Field

[0001] The present invention belongs to the technical field of heterogeneous solid catalysts, and in particular relates to a TiO2 modified carbon nanotube supported palladium single atom catalyst and a preparation method and application thereof. Background Art

[0002] Phenylpropionaldehyde, also known as hydrogenated cinnamaldehyde, is an important fine chemical intermediate used in the production of food flavors and cosmetic fixatives. It is also a key intermediate in the production of HIV therapeutic drugs. Phenylpropionaldehyde synthesis methods include phenylpropionitrile and cinnamaldehyde diethanol acetal. These methods are costly, complex, and pose significant environmental risks. Selective hydrogenation of cinnamaldehyde to produce phenylpropionaldehyde is a green production technology with advantages such as simple process operation, high atomic utilization, environmental friendliness, and readily available raw materials. Cinnamaldehyde is a typical α,β-unsaturated aldehyde with unsaturated C=C and C=O bonds. It is chemically active and prone to oxidation, epoxidation, and polymerization reactions.

[0003] Selective hydrogenation catalysts for cinnamaldehyde often use precious metal catalysts, among which palladium (Pd)-based catalysts show high selectivity for C=C bond hydrogenation, but precious metal resources are limited and the production cost of the catalyst is high. Single atom catalysts (SACs) are considered to be one of the major breakthroughs in modern chemical catalysis due to their ultra-high atom utilization, precise control of the coordination environment, and ultra-high catalytic activity. Currently, SACs have shown excellent catalytic activity and selectivity in catalyzing the selective hydrogenation of unsaturated bonds (such as C≡C, C=C, N=O, C=O, and C=N, etc.). Patent Publication No. CN 115845840 A discloses a graphene-supported atomically dispersed palladium-based catalyst, its preparation method, and application. The atomically dispersed palladium-based catalyst prepared by the graphene-supported palladium-based catalyst can effectively selectively hydrogenate benzaldehyde to produce benzyl alcohol and exhibits good catalytic activity. Therefore, the use of SACs for the selective hydrogenation of cinnamaldehyde to produce phenylpropionaldehyde can effectively improve the utilization rate of precious metals and save costs.

[0004] Although SACs exhibit significantly enhanced activity and selectivity in catalytic selective hydrogenation reactions, the surface free energy of the metal increases dramatically as the particle size decreases to the atomic scale, leading to the aggregation of isolated atoms into nanoparticles during the synthesis process. To improve the cyclic stability and catalytic activity of SACs, the properties of the support are crucial. An ideal support should have the advantages of a high specific surface area and a large number of robust anchoring sites. In addition, by modifying the coordination interactions between the central metal single atom and the coordination species of the solid support, the activity of the catalyst can be maximized by selecting an appropriate support.

[0005] Heterojunctions have attracted interest due to their surprising effects in the field of catalysis due to the synergistic effects between their different components. Because heterojunctions have different energy band structures and surface properties, they can interact with reactants through charge transfer, electron transport, and surface reactions, thereby increasing reaction rates. These interactions can change the adsorption capacity, activation energy, and reaction pathways of the reactants, thereby affecting reaction rates and selectivity. Titanium dioxide (TiO2) is a widely used catalytic material with broad application prospects in environmental purification, water treatment, and new energy fields. Patent publication number CN113634258A discloses a catalyst for photocatalytic reduction of carbon dioxide to produce ethylene and a method for preparing the same. The catalyst prepared is composed of atomically dispersed Cu supported on the surface of a CeO2-TiO2 heterostructure, and can highly selectively reduce CO2 to ethylene under simulated sunlight. Carbon nanotubes (CNTs) have high electrical conductivity and mechanical strength, and TiO2 can be evenly dispersed on them to form a highly dispersed catalyst. Using TiO2 to modify CNTs to form a TiO2-CNT heterojunction can enhance catalyst activity, accelerate reaction rates, and achieve higher catalytic efficiency. It also improves stability: CNTs are highly stable and chemically inert, protecting TiO2 from external influences and extending the life of the catalyst. Furthermore, it increases specific surface area: CNTs have a high specific surface area, allowing for more TiO2 to be loaded onto their surfaces, thereby increasing the catalyst's specific surface area and active sites. Therefore, using TiO2-modified CNTs as supports for metal single atoms effectively improves their dispersion and catalytic activity, resulting in SACs with both high catalytic activity and excellent stability.

[0006] At present, the main methods for preparing SACs include atomic layer deposition, wet chemical method and mass screening-soft landing method, but there are problems such as complex preparation process and high cost. Mechanical activation (MA) is an intense ball milling process and one of the methods for preparing single-atom catalysts. It mainly destroys the dense crystalline structure of the solid material through the impact, shear, friction and other forces generated by the high-speed movement of the ball milling medium, and forms a close interaction between the components. Mechanical activation changes the apparent structure, physicochemical properties and reaction activity of the solid material, which in turn has a positive impact on the subsequent process and product characteristics. It is considered to be a simple and environmentally friendly pretreatment method for preparing functional composite materials. MA has the advantages of high efficiency, simplicity, high reproducibility and high stability. It shows excellent properties in the preparation of composite materials, especially effectively promoting the formation of strong metal-support interactions in composite materials, which undoubtedly provides a new strategy for the preparation of highly stable and highly active single-atom catalyst materials. Summary of the Invention

[0007] In response to the above problems, the present invention provides a TiO2-modified carbon nanotube-loaded palladium single-atom catalyst, a preparation method and application thereof, a hydrothermal synthesis method is used to prepare a TiO2-CNT heterojunction, and a mechanical activation treatment combined with a high-temperature calcination strategy is used to prepare a TiO2 with strong metal-support interaction in situ grown on CNT to form a heterojunction-loaded Pd single-atom catalyst, which is used to efficiently catalyze the selective hydrogenation of cinnamaldehyde to phenylpropionaldehyde.

[0008] The present invention is achieved through the following technical solutions:

[0009] A TiO2-modified carbon nanotube-supported palladium single-atom catalyst is disclosed. TiO2 is in situ grown on CNTs by a hydrothermal synthesis method to form a TiO2-CNT heterojunction. Pd single atoms are then supported on the TiO2-CNT heterojunction by mechanical activation. After high-temperature calcination, the TiO2-modified carbon nanotube-supported palladium single-atom catalyst is obtained.

[0010] Furthermore, the palladium metal content of the TiO2 modified carbon nanotube supported palladium single atom catalyst is 0.18-0.20 wt%, and the specific surface area is 210-215 cm 2 / g.

[0011] A method for preparing the above-mentioned TiO2-modified carbon nanotube-supported palladium single-atom catalyst comprises the following steps:

[0012] (1) CNT pretreatment: CNTs are mixed with concentrated nitric acid for acid washing, and the resulting solution is diluted and centrifuged to neutralize, followed by drying and grinding to obtain pretreated CNTs;

[0013] (2) Preparation of TiO2-CNT heterojunction: The pretreated CNTs were added to anhydrous ethanol and ultrasonically dispersed to obtain solution A, tetrabutyl titanate, glacial acetic acid and anhydrous ethanol were stirred and mixed to obtain solution B, solution B was slowly added to solution A and mixed to obtain solution B, the obtained mixed solution was added to a reactor for oil bath reaction, the generated precipitate was centrifuged and filtered, washed until the supernatant was neutral, and then the obtained solid was freeze-dried to obtain a TiO2-CNT heterojunction;

[0014] (3) Preparation of TiO2-modified carbon nanotube-supported palladium single-atom catalyst: TiO2-CNT heterojunction is mixed with palladium acetate, the resulting mixture is mechanically activated, and then the sieved solid powder is calcined at high temperature under nitrogen or argon protection. After cooling, TiO2-modified carbon nanotube-supported palladium single-atom catalyst is obtained.

[0015] Furthermore, in step (1), the mass ratio of the CNT to concentrated nitric acid is 0.9-1.1 g:100 mL; the temperature of the acid washing is 70-80° C., and the time is 8-9 h.

[0016] Furthermore, in step (2), the mass ratio of the pretreated CNTs to anhydrous ethanol in the solution A is 0.3-0.4 g:20 mL; the mass ratio of tetrabutyl titanate, glacial acetic acid and anhydrous ethanol in the solution B is 3.3-3.5 g:5 mL:20 mL; and the mass ratio of the tetrabutyl titanate to the pretreated CNTs is 8-12:1.

[0017] Furthermore, in step (2), the temperature of the oil bath reaction is 170-190° C., and the time is 10-12 h; and the time of the freeze-drying is 10-12 h.

[0018] Furthermore, in step (3), the mass ratio of the TiO2-CNT heterojunction to palladium acetate is 0.5:0.002-0.005.

[0019] Furthermore, in step (3), the mechanical activation is as follows: adding the mixture and the ball milling medium in a mass ratio of 1:80 to 120 into a mechanically activated solid phase reactor, and ball milling for 1.5 to 2 hours at a rotation speed of 500 to 600 r / min.

[0020] Furthermore, in step (3), the high temperature calcination is: increasing the temperature to 700-800° C. at a rate of 3-5° C. / min and calcining for 1.5-2 hours.

[0021] The invention relates to an application of the above-mentioned TiO2 modified carbon nanotube supported palladium single atom catalyst in catalyzing the selective hydrogenation of cinnamaldehyde to prepare phenylpropionaldehyde.

[0022] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0023] 1. The present invention uses a hydrothermal synthesis method to in situ grow TiO2 on CNTs to form a TiO2-CNT heterojunction. Pd single atoms are then loaded onto the TiO2-CNT heterojunction via mechanical activation. After high-temperature calcination, the resulting TiO2-modified carbon nanotube-supported palladium single-atom catalyst can be used to efficiently catalyze the selective hydrogenation of cinnamaldehyde to phenylpropionaldehyde. This invention successfully constructs a green, economical, stable, and directional Pd single-atom catalyst through mechanical activation combined with a high-temperature calcination strategy. This provides a new method and strategy for the design and development of functional catalysts, effectively promoting the high-value utilization of cinnamaldehyde, and is of great research significance.

[0024] 2. CNTs have high electrical conductivity and mechanical strength. The present invention uniformly disperses TiO2 on them, thereby forming a highly dispersed catalyst. The formation of the TiO2-CNT heterojunction increases the migration and diffusion of carriers between the TiO2 and CNTs, thereby lowering the Fermi level. Due to the synergistic effect of TiO2 and CNTs, the catalyst band gap is reduced, resulting in a mutually embedded structure. This facilitates the stable loading of single atoms of the active component, Pd, on the TiO2-CNT heterojunction support, thereby constructing a supported catalyst with strong metal-support interactions.

[0025] 3. The present invention uses TiO2-CNT heterojunction as a precursor. The strong mechanical action in the mechanical activation treatment can effectively promote the uniform dispersion and embedding of Pd metal in the TiO2-CNT heterojunction, which is conducive to the formation of a strong interaction between Pd metal and TiO2-CNT heterojunction after calcination. The resulting Pd single-atom catalyst has a stable structure and good reusability, thereby improving the efficiency and economic feasibility of the catalyst in practical applications.

[0026] 4. The present invention can refine TiO2-CNT particles through mechanical activation treatment, effectively improving the dispersibility of metal-based components; TiO2 and CNT components are embedded in each other's lattices, and Pd metal is loaded on the heterojunction in the form of single atoms, successfully constructing a stable metal-support structure; mechanical activation treatment strengthens the interfacial interaction between TiO2-CNT components, changes the electron cloud density between them, and gives Pd single atoms a stronger bonding effect. After calcination, it can be stably immobilized on the support, which allows the catalyst to be recycled multiple times.

[0027] 5. The preparation method of the catalyst of the present invention is simple to operate, has readily available raw materials, simple and mild conditions, and low production cost. The catalyst of the present invention has excellent catalytic conversion efficiency for cinnamaldehyde. When the catalyst is used in the process of catalyzing cinnamaldehyde to produce phenylpropionaldehyde, the conversion rate of cinnamaldehyde and the yield of phenylpropionaldehyde are high, with the conversion rate of cinnamaldehyde reaching 99% and the yield of phenylpropionaldehyde reaching 94%. Furthermore, after using the catalyst three times, the conversion rate of cinnamaldehyde reached 98%, and the selectivity of the target product phenylpropionaldehyde also remained above 92%. Therefore, the catalyst has a good application prospect for the efficient conversion of cinnamaldehyde to produce phenylpropionaldehyde. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Pd1 / TiO2-CNT, Pd1 / TiO2, Pd NPs / TiO2-CNT XRD pattern.

[0029] Figure 2 This is the TEM image of Pd1 / TiO2-CNT prepared in Example 1.

[0030] Figure 3 This is the TEM image of Pd1 / TiO2 prepared in Comparative Example 1.

[0031] Figure 4 Pd prepared in Comparative Example 2 NPs TEM image of / TiO2-CNT.

[0032] Figure 5 This is the EDX image of Pd1 / TiO2-CNT prepared in Example 1.

[0033] Figure 6 This is the AC-HAADF-STEM image of Pd1 / TiO2-CNT prepared in Example 1.

[0034] Figure 7 This is the FT-IR graph of Pd1 / TiO2-CNT prepared in Example 1.

[0035] Figure 8 This is the XPS graph of Pd1 / TiO2-CNT prepared in Example 1.

[0036] Figure 9 These are the N2 adsorption-desorption isotherms of Pd1 / TiO2-CNT and Pd1 / TiO2 prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0037] The present invention is further described in detail below through examples. These examples are only used to illustrate the present invention and do not limit the scope of protection of the present invention.

[0038] Example 1

[0039] Preparation of TiO2-modified carbon nanotubes-supported palladium single-atom catalysts:

[0040] (1) CNT pretreatment: 1 g of CNT was mixed with 100 mL of concentrated nitric acid and acid-washed at 80 °C for 8 h. The resulting solution was then diluted and centrifuged to neutralize. After drying and grinding at 60 °C, the pretreated CNT was obtained.

[0041] (2) Preparation of TiO2-CNT heterojunction: 0.3 g of pretreated CNT was added to 20 mL of anhydrous ethanol and dispersed uniformly by ultrasonication for 30 min to obtain solution A. 3.4 g of tetrabutyl titanate, 5 mL of glacial acetic acid, and 20 mL of anhydrous ethanol were stirred and mixed to obtain solution B. Solution B was slowly added to solution A and mixed uniformly. The resulting mixed solution was added to a 100 mL polytetrafluoroethylene reactor and reacted in an oil bath at 180°C for 12 h. The generated precipitate was centrifuged and filtered and washed several times with ethanol and water until the pH of the supernatant was neutral. The obtained solid was then freeze-dried for 12 h to obtain a TiO2-CNT heterojunction.

[0042] (3) Preparation of TiO2-modified carbon nanotube-supported palladium single-atom catalyst: 0.5 g of TiO2-CNT heterojunction and 0.003 g of palladium acetate were added to a ball mill for mechanical activation treatment, and 50 g of zirconium dioxide as a ball milling medium was added. The mixture was ball milled at a speed of 500 r / min for 2 h. After the ball milling was completed, the ball milling medium was separated and the sieved solid powder was calcined at a high temperature of 700 °C at a rate of 3 °C / min in a tubular furnace under nitrogen protection for 2 h. After cooling, TiO2-modified carbon nanotube-supported palladium single-atom catalyst (denoted as Pd1 / TiO2-CNT) was obtained.

[0043] Example 2

[0044] Preparation of TiO2-modified carbon nanotubes-supported palladium single-atom catalysts:

[0045] (1) CNT pretreatment: 1 g of CNT was mixed with 100 mL of concentrated nitric acid and acid-washed at 70 °C for 9 h. The resulting solution was then diluted and centrifuged to neutralize. After drying and grinding at 60 °C, the pretreated CNT was obtained.

[0046] (2) Preparation of TiO2-CNT heterojunction: 0.3 g of pretreated CNT was added to 20 mL of anhydrous ethanol and dispersed uniformly by ultrasonication for 30 min to obtain solution A. 3.5 g of tetrabutyl titanate, 5 mL of glacial acetic acid, and 20 mL of anhydrous ethanol were stirred and mixed to obtain solution B. Solution B was slowly added to solution A and mixed uniformly. The obtained mixed solution was added to a 100 mL polytetrafluoroethylene reactor and reacted in an oil bath at 170°C for 10 h. The generated precipitate was centrifuged and filtered and washed several times with ethanol and water until the pH of the supernatant was neutral. The obtained solid was then freeze-dried for 10 h to obtain a TiO2-CNT heterojunction.

[0047] (3) Preparation of TiO2-modified carbon nanotube-supported palladium single-atom catalyst: 0.5 g of TiO2-CNT heterojunction and 0.002 g of palladium acetate were added to a ball mill for mechanical activation treatment, and 45 g of zirconium dioxide as a ball milling medium was added. The mixture was ball milled at a speed of 500 r / min for 2 h. After the ball milling was completed, the ball milling medium was separated and the sieved solid powder was calcined at a high temperature of 800 °C at a rate of 4 °C / min in a tubular furnace under nitrogen protection for 1.5 h. After cooling, TiO2-modified carbon nanotube-supported palladium single-atom catalyst (denoted as Pd1 / TiO2-CNT) was obtained.

[0048] Example 3

[0049] Preparation of TiO2-modified carbon nanotubes-supported palladium single-atom catalysts:

[0050] (1) CNT pretreatment: 0.9 g of CNT was mixed with 100 mL of concentrated nitric acid and acid-washed at 80 °C for 8 h. The resulting solution was then diluted and centrifuged to neutralize. After drying and grinding at 60 °C, the pretreated CNT was obtained.

[0051] (2) Preparation of TiO2-CNT heterojunction: 0.3 g of pretreated CNT was added to 20 mL of anhydrous ethanol and dispersed uniformly by ultrasonication for 30 min to obtain solution A. 3.3 g of tetrabutyl titanate, 5 mL of glacial acetic acid, and 20 mL of anhydrous ethanol were stirred and mixed to obtain solution B. Solution B was slowly added to solution A and mixed uniformly. The obtained mixed solution was added to a 100 mL polytetrafluoroethylene reactor and reacted in an oil bath at 180°C for 11 h. The generated precipitate was centrifuged and filtered and washed several times with ethanol and water until the pH of the supernatant was neutral. The obtained solid was then freeze-dried for 11 h to obtain a TiO2-CNT heterojunction.

[0052] (3) Preparation of TiO2-modified carbon nanotube-supported palladium single-atom catalyst: 0.5 g of TiO2-CNT heterojunction and 0.004 g of palladium acetate were added to a ball mill for mechanical activation treatment, and 50 g of zirconium dioxide as a ball milling medium was added. The ball milling was carried out at a speed of 600 r / min for 1.5 h. After the ball milling was completed, the ball milling medium was separated and the sieved solid powder was calcined at a high temperature of 800 °C at a rate of 3 °C / min in a tubular furnace under nitrogen protection for 1.5 h. After cooling, TiO2-modified carbon nanotube-supported palladium single-atom catalyst (denoted as Pd1 / TiO2-CNT) was obtained.

[0053] Example 4

[0054] Preparation of TiO2-modified carbon nanotubes-supported palladium single-atom catalysts:

[0055] (1) CNT pretreatment: 1.1 g of CNT was mixed with 100 mL of concentrated nitric acid and acid-washed at 80°C for 9 h. The resulting solution was then diluted and centrifuged to neutralize. After drying and grinding at 60°C, the pretreated CNT was obtained.

[0056] (2) Preparation of TiO2-CNT heterojunction: 0.4 g of pretreated CNT was added to 20 mL of anhydrous ethanol and dispersed uniformly by ultrasonication for 30 min to obtain solution A. 3.3 g of tetrabutyl titanate, 5 mL of glacial acetic acid, and 20 mL of anhydrous ethanol were stirred and mixed to obtain solution B. Solution B was slowly added to solution A and mixed uniformly. The obtained mixed solution was added to a 100 mL polytetrafluoroethylene reactor and reacted in an oil bath at 190°C for 12 h. The generated precipitate was centrifuged and filtered and washed several times with ethanol and water until the pH of the supernatant was neutral. The obtained solid was then freeze-dried for 12 h to obtain a TiO2-CNT heterojunction.

[0057] (3) Preparation of TiO2-modified carbon nanotube-supported palladium single-atom catalyst: 0.5 g of TiO2-CNT heterojunction and 0.005 g of palladium acetate were added to a ball mill for mechanical activation treatment, and 55 g of zirconium dioxide as a ball milling medium was added. The mixture was ball milled at a speed of 600 r / min for 2 h. After the ball milling was completed, the ball milling medium was separated and the sieved solid powder was calcined at a high temperature of 800 °C at a rate of 5 °C / min in a tubular furnace under nitrogen protection for 2 h. After cooling, TiO2-modified carbon nanotube-supported palladium single-atom catalyst (denoted as Pd1 / TiO2-CNT) was obtained.

[0058] Comparative Example 1

[0059] Preparation of TiO2-supported palladium single-atom catalyst:

[0060] (1) Preparation of TiO2: 3.4 g of tetrabutyl titanate, 5 mL of glacial acetic acid, and 20 mL of anhydrous ethanol were stirred and mixed uniformly, then added to a 100 mL polytetrafluoroethylene reactor and reacted in an oil bath at 180°C for 12 h. The resulting precipitate was centrifuged and filtered, washed several times with ethanol and water, and then freeze-dried for 12 h to obtain white TiO2.

[0061] (2) Preparation of TiO2-supported palladium single-atom catalyst: 0.5 g of TiO2 heterojunction and 0.003 g of palladium acetate were added to a ball mill for mechanical activation treatment, and 50 g of zirconium dioxide as a ball milling medium was added. The ball milling was carried out at a speed of 500 r / min for 2 h. After the ball milling was completed, the ball milling medium was separated and the sieved solid powder was calcined at a high temperature of 700 ° C at a rate of 3 ° C / min in a tubular furnace under nitrogen protection for 2 h. After cooling, the TiO2-supported palladium single-atom catalyst (denoted as Pd1 / TiO2) was obtained.

[0062] Comparative Example 2

[0063] The difference between Comparative Example 2 and Example 1 is that the amount of palladium acetate in Comparative Example 2 is 0.006 g, and the other preparation conditions are the same as those in Example 1 to prepare TiO2 modified carbon nanotube supported palladium nanocatalyst (denoted as Pd NPs / TiO2-CNT).

[0064] Material characterization analysis

[0065] (1) XRD analysis

[0066] The Pd1 / TiO2-CNT, Pd1 / TiO2, Pd NPs / TiO2-CNT was characterized and analyzed. The characterization results are as follows Figure 1 shown.

[0067] Depend on Figure 1 It can be seen that the Pd1 / TiO2-CNT sample has crystalline phase peaks at 25.5°, 37.9°, 48.1°, 53.9°, 55.2°, 62.8°, and 68.8°, respectively, which are consistent with the diffraction peaks of the (101), (104), (200), (105), (211), (204), and (116) crystal planes of anatase TiO2. The CNT peak at 26.6° does not appear in the sample, which is due to the overlap with the diffraction of the (101) crystal plane of crystalline TiO2. Therefore, it can be seen that the TiO2 in the prepared Pd1 / TiO2-CNT catalyst is anatase. Anatase TiO2 has a low surface energy and therefore has good hydrophilicity. It has good thermal stability, that is, it has good stability at high temperatures and can maintain the integrity and structural characteristics of the lattice. During the preparation process, surface defects such as oxygen vacancies are easily formed. Oxygen vacancies can increase the number of surface active sites of the catalyst, enhance the adsorption and reaction of adjacent Pd molecules, and these defects create the selective reduction catalytic performance of the catalyst, thereby promoting the catalytic reaction. At the same time, no crystal phase peak of Pd nanoparticles was observed, revealing that the Pd content is low and the dispersion is good. Similarly, in Pd1 / TiO2 and Pd NPs There is no Pd metal related peak in the TiO2-CNT / TiO2-CNT. The possible reasons are: (1) the Pd metal content is too low; (2) the Pd metal is well dispersed in the TiO2-CNT heterojunction.

[0068] (2) TEM analysis

[0069] Transmission electron microscopy (TEM) was used to examine the Pd1 / TiO2-CNT, Pd1 / TiO2, PdNPs / TiO2-CNT were characterized and analyzed. The characterization results were as follows: Figure 2-4 shown.

[0070] Depend on Figure 2 (a) It can be seen that TiO2 nanoparticles are evenly attached to the tubular carbon nanotubes; Figure 2 (c) shows that the lattice fringes of 0.351 and 0.947 nm correspond to the (101) plane of TiO2, confirming that TiO2 is formed on the surface of carbon nanotubes, and no Pd metal aggregation is observed. The Pd1 / TiO2-CNT composite material shows a clear interface between carbon nanotubes and TiO2, indicating the successful formation of heterojunction structure. It can be seen that the hollow and tubular structures confirm the existence of CNTs. Similarly, Figure 3 It can be seen that no Pd metal aggregation was found on the surface of TiO2 nanoparticles in the Pd1 / TiO2 catalyst. Figure 4 It can be seen that Pd NPs There are multiple Pd metals aggregated into nanoclusters in TiO2-CNT, indicating that the excessive Pd metal content leads to the aggregation of metals, which is also the NPs The reason why the catalytic effect of / TiO2-CNT is far inferior to that of Pd1 / TiO2-CNT.

[0071] (3) EDX analysis

[0072] Energy dispersive X-ray spectroscopy (EDX) was used to characterize the Pd1 / TiO2-CNT prepared in Example 1. Figure 5 As shown. Figure 5 (ae) The Pd1 / TiO2-CNT catalyst contains four elements: C, O, Ti, and Pd, demonstrating the successful loading of Pd onto the support. Furthermore, the Pd element is uniformly dispersed, with no Pd nanoparticles observed.

[0073] (IV) AC-HAADF-STEM analysis

[0074] The Pd1 / TiO2-CNT prepared in Example 1 was characterized and analyzed using a spherical aberration corrected high-angle annular dark field scanning transmission microscope (AC-HAADF-STEM). Figure 6 As shown. Figure 6 It can be seen that Pd1 / TiO2-CNT clearly shows single atomic points marked with yellow circles. The figure shows that Pd single atoms are mainly distributed on TiO2, and a small part is dispersed on CNT, which is consistent with the EDX characterization results.

[0075] (5) FT-IR analysis

[0076] The Pd1 / TiO2-CNT prepared in Example 1 was characterized and analyzed using Fourier transform infrared spectroscopy (FT-IR). Figure 7 As shown. Figure 7 It can be seen that the broadband of CNT-TiO2 heterojunction composed of Ti-O-Ti and Ti-OC bonds appears at 500 cm -1 In addition, at 1720, 1550 and 1370 cm -1 The peaks on the left and right are attributed to the C=O carbon group, the C=C skeletal vibration of the carbon nanotube, and the CO carboxyl group, respectively. The weakening of the CO peak also indicates in situ growth of TiO2 on the CNT. The formation of these peaks indicates a good bonding between the carbon nanotube and titanium dioxide. The reduced C=C wavenumber in the Pd1 / TiO2-CNT is likely due to the enhanced conjugation of the carbon nanotubes through the formation of the TiO2-CNT heterojunction.

[0077] (6) XPS analysis

[0078] The surface chemical state and elemental composition of the Pd1 / TiO2-CNT prepared in Example 1 were characterized by X-ray photoelectron spectroscopy (XPS). Figure 8 As shown. Figure 8 (a) It can be seen that the Pd1 / TiO2-CNT catalyst has C, O, Ti and Pd elements at the same time. Figure 8 (b) It can be seen that there are three fitting peaks in the XPS spectrum of C1s in the catalyst. The peak at the binding energy of 284.8 eV is the CC bond, the peak at the binding energy of 285.2 eV is the COC bond, and the peak at the binding energy of 288.9 is the CO or Ti-O bond. Among them, the fitting peak area of ​​the COC bond is higher than that of other C bonds, which indicates that the C bonds in the Pd1 / TiO2-CNT catalyst mainly exist in the form of CO bonds.

[0079] Depend on Figure 8 (c) It can be seen that there are two fitting peaks in the XPS spectrum of O1s in the catalyst. The binding energy at 531.1eV is lattice oxygen, that is, Ti-O or CO bond. The binding energy at 532.8eV is Ti-OC bond, which confirms the in-situ growth of TiO2 on CNT to form a heterojunction. In addition, a small amount of vacant oxygen appears at 529.4eV, providing a more stable adsorption site for Pd. Catalyst XPS shows that oxygen is mainly lattice oxygen, which indicates that oxygen atoms have chemical bonds with the catalyst lattice, and this chemical bond is relatively strong. In addition, this also shows that there are relatively few adsorbed oxygen species on the catalyst surface, which may provide more reaction sites for reactants and promote the reaction.

[0080] From Figure 8(d), we can see that there are three fitting peaks in the XPS spectrum of Ti 2p in the catalyst, with binding energies at 459.8, 459.6 and 462.1 eV, respectively. 4+ 、Ti 3+ and Ti 2+ There is a fitting peak at 472.9eV. From 4e, we can see that there are four groups of fitting peaks in the XPS spectrum of Pd 3d in the catalyst, and the binding energy is at 336.2 and 342.1eV respectively. 0 3D 5 / 2 and 3D 3 / 2 The fitting peaks of PdO are at 338.6 and 345.1 eV, respectively. x (x≤1)Pd x+ 3D 5 / 2 and 3D 3 / 2 The fitting peaks of PdO are at 345.1 and 351.8 eV, respectively. x (1 <x≤2)Pd x+ 3D 5 / 2 and 3D 3 / 2 Therefore, the Pd single atom is mainly in the form of reduced Pd 0 and oxidation state Pd x+ Two forms exist.

[0081] (7) ICP-OES analysis

[0082] The Pd1 / TiO2-CNT prepared in Example 1 was characterized and analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES), and it was found that the palladium metal content in the Pd1 / TiO2-CNT catalyst was 0.19 wt%.

[0083] (8) Specific surface area analysis

[0084] The Pd1 / TiO2-CNT and Pd1 / TiO2 prepared in Example 1 and Comparative Example 1 were characterized and analyzed by N2 adsorption-desorption. The characterization results are shown in the figure. Figure 9 As shown. Figure 9 It can be seen that the specific surface area of ​​Pd1 / TiO2-CNT catalyst is 212 cm 2 / g, while the specific surface area of ​​Pd1 / TiO2 is only 12.2cm 2 / g, which is due to the addition of CNTs, which greatly improves the specific surface area of ​​Pd1 / TiO2-CNT catalyst.

[0085] Application Example 1

[0086] The Pd1 / TiO2-CNT catalyst prepared in Example 1 was placed in a hydrothermal reactor in an autoclave, along with 0.5 mmol of cinnamaldehyde, 5 mL of acetonitrile, and 20 mg of the catalyst. The reactor was then filled with 1 MPa of H2 and placed in a 60°C oil bath. After reacting for 3 hours, the reactor was removed from the oil bath and rapidly cooled in a water bath to terminate the reaction. The resulting reaction solution was cooled and filtered through a 0.22 μm pore size filter to obtain phenylpropionaldehyde. The conversion of cinnamaldehyde was determined to be 38%, and the yield of phenylpropionaldehyde was 88%.

[0087] Application Example 2

[0088] The difference between Application Example 2 and Application Example 1 is that the oil bath temperature in Application Example 2 is 70° C., and the other preparation conditions are the same as those in Application Example 1. The conversion rate of cinnamaldehyde was 36%, and the yield of phenylpropanal was 93%.

[0089] Application Example 3

[0090] The difference between Application Example 3 and Application Example 1 is that the oil bath temperature in Application Example 3 is 80° C., and the other preparation conditions are the same as those in Application Example 1. The conversion rate of cinnamaldehyde was 99%, and the yield of phenylpropionaldehyde was 94%.

[0091] Application Example 4

[0092] The difference between Application Example 4 and Application Example 1 is that the oil bath temperature in Application Example 4 is 90° C., and the other preparation conditions are the same as those in Application Example 1. The conversion rate of cinnamaldehyde was 88%, and the yield of phenylpropanal was 85%.

[0093] Application Example 5

[0094] The difference between Application Example 5 and Application Example 1 is that the oil bath temperature in Application Example 5 is 100° C., and the other preparation conditions are the same as those in Application Example 1. The conversion rate of cinnamaldehyde was 86%, and the yield of phenylpropionaldehyde was 80%.

[0095] Application Example 6

[0096] The difference between Application Example 6 and Application Example 3 is that the H2 pressure in Application Example 6 is 0.5 MPa, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 97%, and the yield of phenylpropionaldehyde was 79%.

[0097] Application Example 7

[0098] The difference between Application Example 7 and Application Example 3 is that the H2 pressure in Application Example 7 is 1.5 MPa, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 85%, and the yield of phenylpropanal was 85%.

[0099] Application Example 8

[0100] The difference between Application Example 8 and Application Example 3 is that the reaction time in Application Example 8 is 1 h, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 11%, and the yield of phenylpropanal was 89%.

[0101] Application Example 9

[0102] The difference between Application Example 9 and Application Example 3 is that the reaction time in Application Example 9 is 2 h, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 87%, and the yield of phenylpropanal was 91%.

[0103] Application Example 10

[0104] The difference between Application Example 10 and Application Example 3 is that the reaction time in Application Example 10 is 4 h, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 99%, and the yield of phenylpropanal was 83%.

[0105] Application Example 11

[0106] The difference between Application Example 11 and Application Example 3 is that the reaction time in Application Example 11 is 5 h, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 99%, and the yield of phenylpropanal was 84%.

[0107] Application Example 12

[0108] The difference between Application Example 12 and Application Example 3 is that the amount of catalyst used in Application Example 12 is 10 mg, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 38%, and the yield of phenylpropanal was 83%.

[0109] Application Example 13

[0110] The difference between Application Example 13 and Application Example 3 is that the amount of catalyst used in Application Example 13 is 15 mg, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 75%, and the yield of phenylpropanal was 78%.

[0111] Application Example 14

[0112] The difference between Application Example 14 and Application Example 3 is that the amount of catalyst used in Application Example 14 is 25 mg, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 99%, and the yield of phenylpropanal was 76%.

[0113] Application Example 15

[0114] The difference between Application Example 15 and Application Example 3 is that the amount of catalyst used in Application Example 15 is 30 mg, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 99%, and the yield of phenylpropanal was 74%.

[0115] Application Example 16

[0116] Application Example 16 differs from Application Example 3 in that the catalyst in Application Example 16 is the catalyst used three times in Application Example 3, and the remaining preparation conditions are the same as those in Application Example 3. Measurements showed that the conversion rate of cinnamaldehyde was 98%, and the yield of phenylpropanal was 92%.

[0117] Comparative Application Example 1

[0118] The difference between Application Example 1 and Application Example 3 is that the catalyst in Application Example 1 is the Pd1 / TiO2 catalyst prepared in Comparative Example 1, and the other preparation conditions are the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 37%, and the yield of phenylpropanal was 83%.

[0119] Application Comparative Example 2

[0120] The difference between the comparative example 2 and the example 3 is that the catalyst in the comparative example 2 is the Pd NPs / TiO2-CNT catalyst, and the other preparation conditions were the same as those in Application Example 3. The conversion rate of cinnamaldehyde was 55%, and the yield of phenylpropanal was 88%.

[0121] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A TiO2-modified carbon nanotube-supported palladium single-atom catalyst, characterized in that: TiO2 was in situ grown on CNTs using a hydrothermal synthesis method to form a TiO2-CNT heterojunction. Pd single atoms were then loaded onto the TiO2-CNT heterojunction through mechanical activation. After high-temperature calcination, TiO2-modified carbon nanotube-loaded palladium single-atom catalyst was obtained. The palladium metal content of the TiO2-modified carbon nanotube-supported palladium single-atom catalyst is 0.18-0.20 wt % and the specific surface area is 210-215 cm 2 / g.

2. A method for preparing a TiO2-modified carbon nanotube-supported palladium single-atom catalyst as claimed in claim 1, characterized in that: The following steps are involved: (1) CNT pretreatment: CNTs are mixed with concentrated nitric acid for acid washing, and the resulting solution is diluted and centrifuged to neutralize. After drying and grinding, the pretreated CNTs are obtained. (2) Preparation of TiO2-CNT heterojunction: The pretreated CNTs were added to anhydrous ethanol and ultrasonically dispersed to obtain solution A. Tetrabutyl titanate, glacial acetic acid and anhydrous ethanol were stirred and mixed to obtain solution B. Solution B was slowly added to solution A and mixed to obtain solution B. The obtained mixed solution was added to a reactor for oil bath reaction. The generated precipitate was centrifuged and filtered, washed until the supernatant was neutral, and then the obtained solid was freeze-dried to obtain a TiO2-CNT heterojunction. (3) Preparation of TiO2-modified carbon nanotube-supported palladium single-atom catalyst: TiO2-CNT heterojunction is mixed with palladium acetate, the resulting mixture is mechanically activated, and then the sieved solid powder is calcined at high temperature under nitrogen or argon protection. After cooling, TiO2-modified carbon nanotube-supported palladium single-atom catalyst is obtained.

3. The method for preparing the TiO2-modified carbon nanotube-supported palladium single-atom catalyst according to claim 2, wherein: In step (1), the mass volume ratio of the CNT to concentrated nitric acid is 0.9-1.1 g: 100 mL; the temperature of the pickling is 70-80° C., and the time is 8-9 h.

4. The method for preparing the TiO2-modified carbon nanotube-supported palladium single-atom catalyst according to claim 2, wherein: In step (2), the mass volume ratio of the pretreated CNTs to anhydrous ethanol in the solution A is 0.3-0.4 g:20 mL; the mass volume ratio of tetrabutyl titanate, glacial acetic acid and anhydrous ethanol in the solution B is 3.3-3.5 g:5 mL:20 mL; and the mass ratio of tetrabutyl titanate to the pretreated CNTs is 8-12:

1.

5. The method for preparing the TiO2-modified carbon nanotube-supported palladium single-atom catalyst according to claim 2, wherein: In step (2), the temperature of the oil bath reaction is 170-190° C., and the time is 10-12 h; the time of the freeze-drying is 10-12 h.

6. The method for preparing the TiO2-modified carbon nanotube-supported palladium single-atom catalyst according to claim 2, wherein: In step (3), the mass ratio of the TiO2-CNT heterojunction to palladium acetate is 0.5:0.002-0.

005.

7. The method for preparing the TiO2-modified carbon nanotube-supported palladium single-atom catalyst according to claim 2, wherein: In step (3), the mechanical activation is as follows: adding the mixture and the ball milling medium in a mass ratio of 1:80 to 120 into a mechanically activated solid phase reactor, and ball milling for 1.5 to 2 hours at a rotation speed of 500 to 600 r / min.

8. The method for preparing the TiO2-modified carbon nanotube-supported palladium single-atom catalyst according to claim 2, wherein: In step (3), the high temperature calcination is: increasing the temperature to 700-800°C at a rate of 3-5°C / min and calcining for 1.5-2 h.

9. Use of the TiO2-modified carbon nanotube-supported palladium single-atom catalyst as claimed in claim 1 in catalyzing the selective hydrogenation of cinnamaldehyde to prepare phenylpropionaldehyde.

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